Recently, in attempting to provide safety means for the preparation of drivers and passengers of automotive vehicles and the like, a variety of systems have been developed for the control of a travel interval distance between two vehicles for the prevention of an accidental collision or contact. Such systems provide an electronic device such as a radar system adapted to be installed on the vehicle for the purpose of measuring a distance from the vehicle to another existing in the traffic or approaching toward the path of that vehicle or an obstacle on the road, and/or a relative velocity therewith, and when the vehicle comes within a dangerously close region wherein there is a possibility of collision or contact, the driver is then warned or the vehicle's brake is actuated, so as to eventually prevent a possible collision or contact from occurring.
Among such radar systems, commonly known types are: the two-frequency Doppler radar system adapted to measure an interval distance to an object and/or a relative velocity therewith by way of the Doppler frequency shift generated by a relative velocity from a radio wave beam emitted originally against an object and a reflected-to-return wave therefrom, the pulse radar system for measuring a distance to the object by determining a wave propagation time in a round or turnaround trip to and from the object, and the FM-CW type radar system for determining a distance to the objects from a beat frequency generated from a phase shift between an emitted wave and receiving waves. While the two-frequency Doppler radar system suffers from the disadvantage that it cannot function with respect to two objects having no relative velocity therebetween, the pulse radar system and the FM-CW type radar system can determine a distance between two objects with a zero relative velocity. Consequently, it is of course the latter systems that can be adapted to an object which is stationary or an obstacle where there is no relative velocity with respect to the moving source of emission of radio waves.
Furthermore, because the pulse radar system cannot practicably measure a distance smaller than the output pulse width from the system, it is essential to have an extremely small output pulse width to practice the measuring of a relatively short distance required. In view of such disadvantageous requirement, such a pulse radar system has an attendant technical difficulty in the design per se and in the technique of receiving reflected-to-return waves as well. In this respect, the FM-CW type radar system is in contrast advantageous in its relatively simple construction as well as its excellent controllability. Moreover, it can readily perform the measurement of an extremely small distance when compared with the pulse radar system.
To provide a better understanding of the present invention, the following is a general explanation of the fundamental principle of operation of the so-called FM-CW type radar system, to which the present invention is directed.
As typically shown in FIG. 1, when a radio wave D.sub.1, having its oscillating frequency f.sub.o modulated by increasing to a level of f.sub.o +.DELTA.f and then descreasing back to f.sub.o at a time interval or cycle of Tm, is emitted against an object and then a reflected wave D.sub.2 is received, there is obtained a phase shift between the two waves by a propagation time of t.sub.R to and from the object on the same timing basis (see the Equation 1 hereinbelow). With such phase shft, there is produced a beat frequency f.sub.R, and the thus-obtained beat frequency f.sub.R is proportional to a propagation time t.sub.R or a distance X to and from the object as represented in the Equation 2 below, and consequently, it is thus possible to determine the distance X by measuring the magnitude of the beat frequency f.sub.R. On the other hand, by obtaining a time lapse at the distance X to the object detected, a relative velocity Vr can now be determined from the Equation 3, below. EQU t.sub.R =2X/c . . . (1)
where, c represents a propagation velocity of a wave. EQU f.sub.R =(2/Tm).multidot..DELTA.f.multidot.t.sub.R . . . (2) EQU dX/dt=Vr . . . (3)
Now, referring to FIG. 2, there is shown the conventional FM-CW type radar system specifically designed to be installed on an automotive vehicle so that it may operate to detect an interval distance and a relative velocity between that vehicle and an object approaching or interfering therewith, to determine an appropriate or safety vehicle interval between the two in accordance with a predetermined function on the basis of the result of detection and the existing velocity signal of that vehicle, and to then decide in accordance with comparison of the thus-obtained safety interval and the existing actual interval distance between the two to produce a command as necessary for warning the driver of that vehicle or a command for actuating the brake of the vehicle, accordingly. More particularly, this FM-CW type radar system for use in the automotive vehicle is constructed to provide the steps of modulating the oscillating frequency f.sub.o from an oscillator 1 to a given frequency with a modulator 2, emitting the thus-modulated output from an antenna 5 through a directional coupler 3 and a circulator 4 to the object, receiving a reflected-to-return wave from the object and sending it through the circulator 4 to a mixer 6, mixing this reflected wave with a transmitted wave branched from the above mentioned directional coupler 3 so as to obtain beat frequencies f.sub.R, amplifying the thus-obtained small beat frequency signals with a video signal amplifier 7 to a desired voltage level, thereafter sending thus-amplified signals to a frequency counter 8 so as to read out the frequencies thereof, and then supplying the thus-read value of the beat frequency to a signal processor 9 so as to determine the vehicle interval X and relative velocity Vr from the arithmetic operation set forth above. Concurrently, the existing appropriate safety vehicle interval Xs is determined in accordance with the predetermined function wherein the appropriate safety vehicle interval is previously stored with respect to an existing velocity signal Vs provided by a speedometer of the vehicle or the like and the calculated relative velocity Vr with respect to the object, the thus-obtained safety vehicle interval Xs is then compared with the existing actual interval X, and if Xs&lt;X, a warning to the driver of the vehicle or a command for actuating the vehicle's brake is produced.
According to the conventional FM-CW type radar system of typical construction as described above, it is very possible that the radar system may receive ghost signals upon which it would operate improperly, thus eventually degrading the performance of the system. More specifically, when there are a plurality of objects in the traffic around a vehicle in question, i.e., when there are two other vehicles A and B running in front of and at different respective distances from vehicle C in question equipped with the radar system, as exemplified in FIG. 3, the radar system would receive reflected waves from both vehicles A and B at the same time, thus producing beat frequencies from the mixture of two receiving waves from these vehicles. Under such circumstance, it would consequently be impossible to distinguish between vehicle A and vehicle B in the traffic. Incidentally, because there is actually a limit in the directivity of a radio wave beam to be emitted from the antenna installed on a vehicle, some of the reflected waves from the road surface or the like may very possibly be received by the antenna, even if it is substantially flat, thus resulting in beat frequencies from such reflected waves so as to produce possible phantom or ghost signals to show false images which really do not exist. On the other hand, if the conventional FM-CW type radar system was provided with an improved sensitivity of its receiver, it would naturally become too sensitive to the reflected waves from the road surface or the like, thus making it impossible to decide which is a correct and true signal.
In consideration of the above-discussed inevitable drawbacks or disadvantages inherent in the types of radar systems other than the FM-CW type radar system, as well as those of the conventional FM-CW type radar system, it would undoubtedly be advantageous to make the best use of the fundamental advantageous features of the FM-CW type radar system over others, yet avoiding the undesired shortcomings thereof.